Since a lithium-ion secondary battery, one type of nonaqueous electrolyte secondary batteries, is light in weight and has a large charging and discharging capacity, it has been used mainly as a battery for portable electronic devices. Moreover, practical use of lithium-ion secondary batteries as batteries for motor vehicles such as electric automobiles is expected. Generally, materials comprising a rare metal such as cobalt or nickel are used as a positive-electrode active material of a lithium-ion secondary battery. However, due to the fact that rare metals are small in the distributed amount, not always easily available and additionally expensive, a positive-electrode active material using a material that replaces a rare metal has been required.
A technique of using elemental sulfur as a positive-electrode active material is known. Namely, sulfur is easily available compared to rare metals and is inexpensive, and has a further advantage that a charging and discharging capacity of a lithium-ion secondary battery can be made larger than the present state. For example, it is known that a lithium-ion secondary battery using sulfur as a positive-electrode active material can achieve about 6 times larger charging and discharging capacity than a lithium-ion secondary battery using lithium cobalt oxide which is a general positive-electrode material.
However, the lithium-ion secondary battery using elemental sulfur as the positive-electrode active material has a problem that a battery capacity is deteriorated through repeated charging and discharging. That is, elemental sulfur likely generates a compound with lithium when discharging and since the generated compound is soluble into a nonaqueous electrolyte (for example, ethylene carbonate and dimethyl carbonate and the like) of the lithium-ion secondary battery, the charging and discharging capacity is gradually reduced through repeated charging and discharging due to the sulfur eluting into the electrolyte.
There is a technique of using polyacrylonitrile as a starting material for a sulfur-based positive-electrode active material. However, polyacrylonitrile is a relatively expensive material. Further, in a lithium-ion secondary battery using this positive-electrode active material for a positive electrode, battery performances such as a charging and discharging capacity and cyclability greatly depend on a quality (particularly a particle size) of a staring polyacrylonitrile powder. Polyacrylonitrile having good quality is further expensive. Therefore, it is difficult to provide an inexpensive lithium-ion secondary battery having a large charging and discharging capacity and excellent cyclability.
It is known that in order to improve cyclability by preventing sulfur from eluting into an electrolyte, a sulfur-based positive-electrode active material prepared by heat-treating sulfur together with a diene rubber is to be mixed with a fluorine-containing resin as a binder and the mixture is used as a positive-electrode material (WO 2015/050086). However, sufficient cyclability has not yet been obtained.